In the metallurgy, chemical, and mechanical industries, it is required to use electric furnaces. Electric furnace transformer is a transformer used for supplying power to an electric furnace in these industries, and it is used for reducing a higher grid voltage to an operation voltage that the electric furnace requires. Because the secondary output voltage of the electric furnace transformer is rather low (from tens of volts to hundreds of volts), the secondary output current is normally large, it can be up to tens of thousands of amperes or even hundreds of thousands of amperes. Another feature of the electric furnace transformer is that the regulating range of the secondary voltage is large, and sometimes, it is required that the secondary voltage can be regulated from the maximum value to 25%˜50% of the maximum value. So the essential difference exists between the electric furnace transformer and the power transformer.
For the voltage regulation of the power transformer, the fluctuation of the grid voltage can be adapted by regulating the number of the turns of the primary winding so that the secondary voltage can be maintained constant. The flux of the transformer core is unchanged when regulating the voltage. Such voltage regulation method is called constant flux voltage regulation.
For the voltage regulation of the electric furnace transformer, the secondary voltage is changed under the condition that the primary voltage is unchanged. Because the secondary voltage is low and the number of turns of the winding is small, the voltage regulation tap cannot be disposed at the secondary side to perform the constant flux voltage regulation. In order to regulate the secondary voltage of the electric furnace transformer, there are normally three types of voltage regulation methods based on the practical conditions: direct voltage regulation through varying flux; voltage regulation through transformers connected in series; and voltage regulation through autotransformer. The direct voltage regulation through varying flux is used in the condition that the level difference is not required for the voltage regulation and the regulation range is small. The voltage regulation through transformers connected in series and the voltage regulation through autotransformer are used for a large voltage regulation range or a voltage regulation with level difference.
For example, according to an example, the input voltage at the primary side of a large-capacity (65 MVA) single-phase calcium carbide furnace transformer is 110 kV, it is required that the impedance value of the transformer is small, and it is also required that the impedance for the maximum output voltage tap is less than 3.5%, the maximum output current at the low voltage side is 155 kA, and the output voltage at the low voltage side is 500˜1000 V. Because of the impedance and the limitations of the transportation, the autotransformer is required in this example, and the principle of the conventional design is shown as FIG. 1a, and the structural arrangement is shown as FIG. 1b. As shown in the figures, a single-phase transformer group 100 consists of a secondary autotransformer 101 having regulating winding and a main transformer 102 having fixed transformer ratio. The autotransformer 101 comprises a voltage regulating autotransformer winding 105, and the main transformer 102 comprises a main transformer winding 106. Two individual magnetic cores of the transformer 101 and the transformer 102 form the magnetic circuit portion (voltage regulating autotransformer core 103 and main transformer core 104). In order to reduce the impedance and the transportation height, the main transformer has two winding columns, connecting in series at the primary side, and connecting in parallel at the secondary side. The impulse voltage of the electric grid of 110 kV at the primary side effects directly at the on-load switch and regulating winding to produce large oscillating voltage, the voltage between two terminals at the primary side of the main transformer and between terminals to earth may largely increase.